General Principles of Astronomy

To determine where on the horizon the sun, the moon or a star will rise on a particular day, it is only necessary to know three things.

  • The Latitude of the observer on the earths surface.
  • The Altitude - the Angle of Horizon above the horizontal plane through the observer’s eyes.
  • The Declination of the Celestial Body on that day.

These three things - which will be slightly modified by such items as refraction and the radius of the solar or lunar disc - are all angles which, when numerically known, enable a spherical triangle to be solved to give a fourth factor:

the angle between the vertical plane through the celestial body and the plane of the meridian through the observer. Simply it provides the angle which the line of observation to the celestial body makes with the line of true north.